1. Field of the Invention
The invention relates to a voltage detection and sequencing circuit.
2. Description—Related Art
Many electrical systems require power to be applied to subsystems and components in a predetermined time sequence during system power up. This is particularly the case with computer systems where supply voltages must be applied to different subsystems in a predetermined order when the computer is turned on. For example, complex system on chip (SOC) systems such as the Ethernet switch SOC require at least two power supplies to power up the chip. Some SOC chips even require three or four power supplies to power the chip. In the Ethernet switch SOC, two different types of power supply are required, namely an Input/Output (IO) power supply of 3.3V and a core voltage supply of 1.8V. The IO power supply provides the necessary power for interface with other chips while the core voltage is used to power the chip's core. If the power is not supplied to the chip in the correct sequence, namely the core voltage supply must be supplied before the IO supply voltage, then the chip will not operate and the user of a system incorporating such a chip will encounter problems. The problems become worse when more than two power supplies have to be sequenced.
Voltage sequencing circuits have, therefore, been developed to sequence the application of voltages in such systems and to control the delay period between the application of two successive voltages. An example of a prior art voltage sequencing circuit is shown in FIG. 3, which illustrates such a circuit for sequencing the power supply to an SOC such as the aforesaid Ethernet switch SOC. Here, the chip 10 is located within an external power sequencing circuit and has a number of pins 1 to 8 to provide communication between the power sequencing circuit and the rest of the electrical system. A core voltage of 1.8 V is supplied to the chip 10 via voltage terminals 11, 12, and resistors 14 and 15 are arranged with a trip point of 1.65 V. An IO power supply of 3.3V is supplied to the chip 10 via voltage terminals 16, 17 and a pass device 18, such as an insulated gate field effect transistor (IGFET) that has source-drain terminals connected to the voltage terminals 16, 17 and a gate terminal connected to the chip 10. The chip 10 monitors the core voltage 11, 12 and will only turn on the pass device 18 when the core voltage supply reaches a threshold of 1.65V. When the pass device is turned on, the IO supply voltage of 3.3V is then supplied to the chip 10. Hence, the voltage sequencing circuit ensures that the IO supply voltage is supplied later than the 1.8 core voltage.
It will be appreciated, however, that this arrangement has several disadvantages. The primary disadvantage is that it introduces complexity into the electrical system because of the external circuit components. This complexity is significantly increased in cases where a third power supply needs to be sequenced. Also, the cost of the system is increased as a direct result of the complexity as additional components must be added and space is required to accommodate them. This also adds to the design costs. Accordingly, there is a need for simplified and cost effective voltage detection and sequencing circuit.